Data transmission systems are known and are used to exchange information and conduct transactions with remotely positioned portable data devices. Such portable data devices are commonly referred to as cards, smartcards, or, in a simplified embodiment, tags. Likewise, data transmission terminals are commonly referred to as exciters or card readers. In such a data transmission system, a transaction begins when a card enters the excitation field of the terminal/reader. In particular, the terminal produces a power signal as well as a data signal (referred to as an information signal), and transmits this information signal using a carrier signal. It is the detection and reception of this carrier signal that remotely powers the card, and enables the card circuitry to perform its intended function.
It is well known that remotely powered (i.e., contactless) data devices (i.e., cards) can be used to perform a variety of tasks, including theft prevention, personnel or material identification and control, automatic fare collection, money and service transaction recording and control, and the like. While identification tags may be less complex, so-called "smartcards" tend to be more complex and may include one or more processors, as well as local memory to store and process information. Accordingly, the carrier signal transmitted by the terminal must be modulated to transfer data from the reader to the card, and detected to provide a source of power for the active circuits.
To meet the requirements of today's smartcard applications, the modulation applied to the signal sent from the reader to the card should have minimal modulation sideband spectral content, contain a DC component of high and consistent value, and be detectable with minimal card complexity. Several approaches used to date, as next described, fail to meet these basic requirements in one or more regards.
One example of such a modulation scheme is the pulse width modulation (PWM) system disclosed in U.S. Pat. No. 5,345,231 assigned to Mikron Gesellschaft for Integrierte Mikroelectronik mbH and issued Sep. 6, 1994. This system provides for the signal from the sending station (terminal) at frequency f.sub.0 being pulse-width modulated according to the binary source data to be transmitted to the card. That is, a binary "0" of the source data is encoded into a burst of carrier frequency f.sub.0 having time duration t.sub.0, and binary "1" of the source data is encoded into a burst of carrier frequency f.sub.0 having time duration t.sub.1. Carrier frequency f.sub.0 is absent for a predetermined time period between bursts to provide a delimiter between adjacent symbols.
A second example of the prior art is the modified Miller-encoded system used in the so-called Mifare.TM. product manufactured by the Philips Corporation. This system provides for the signal from the sending station (terminal) at carrier frequency f.sub.0 to be modulated according to the binary source data to be transmitted to the card in the following way: a binary "0" is encoded into a symbol having the same duration as a source data bit and including a cessation in the transmission of carrier frequency f.sub.0 for a predetermined time period beginning at the start of a bit; a binary "1" is encoded into a symbol having the same duration as a source data bit and including a cessation in the transmission of carrier frequency f.sub.0 for a predetermined time period beginning at the middle of a bit; and the first binary "0" following a binary "1" is encoded into a symbol having a continuous transmission of carrier frequency f.sub.0 for the entire duration of a source bit.
There are several problems with the prior art, including high modulation sideband emission levels, and a DC component that is diminished by discontinuous transmission and modulated significantly according to the binary source data sequence.
For illustration, FIG. 1 shows the spectrum of a signal transmitted from the terminal with a random data input and using the modified Miller approach, as described above. The spectrum shows the carrier 102, and modulation sideband components that occur at discrete frequencies 106, 107, 108, and 109, and over a continuum of frequencies 104 due to the random nature of the data input. For this modulation, the difference between the level of the carrier and the modulation sideband components is determined to be level difference 110.
The rules governing the level and structure of modulation sideband components generated by the terminals used with cards, smartcards, or tags may vary from country to country. In general, the measured radiated field strength of the modulated sideband components must lie below a set limit to meet compliance requirements in a given country. A transmitted signal containing discrete spectral components will not benefit from a reduced measured field strength level due to frequency selectivity in the measuring receiver. Thus, the power transmitted by the terminal and, in turn, the range within which the terminal and portable data device can reliably interact, may be restricted when such discrete components are present in the signal.
Further, techniques such as PWM are difficult to implement in a synchronous processing system, such as a microprocessor embodiment, since the duration of the symbol for a binary "1" is longer than the duration of the symbol for a binary "0". For these reasons, an improved data transmission terminal and contactless data/power delivery method for use therewith is required.